No description available.

No description available.

No description available.

Previous investigations have revealed that some polymer solutions exhibit dielectric dispersion (variation of the dielectric constant with frequency) which is apparently related in some way to the molecular weight of the polymer (1) (2) (3). One of these investigations (1) was carried out on solutions to cellulose acetate in dioxane. It was found that the change in dielectric constant was so small that rather concentrated solutions had to be used in order to obtain reproducible results. The possibility of intermolecular interactions in concentrated solutions made dilute solution measurements desirable. Consequently, the object of the present investigation was to carry out such measurements with sufficient precision to define the shape of the dielectric dispersion curve over the necessary frequency range. The fundamental difficulty was that the change in dielectric constant of a solution containing less than 1% cellulose acetate was only about one part in five hundred or less in a frequency range of about three decades. The course ot such a small change is difficult to detect over such a wide band of frequencies. Capacitance bridges; which are very good for making measurements at a single frequency, particularly in the range of frequencies under investigation; were commercially available. However, bridges have certain inherent disadvantages which make difficult the coverage of such a wide band of frequencies with the necessary precision. The use of ordinary resonance methods was desirable, but they are limited by the practical sizes of the components to frequencies well above the lowest frequencies which were expected to be encountered. As a result of these limitations, a modified resonance procedure was adopted. A resistance-capacitance oscillator was substituted for the more conventional inductance-capacitance circuits. Null indications were obtained by substituting a frequency counting system of great precision for the "zero beat" method. It had been found earlier that the critical frequency was related to the weight average molecular weight of the polymer sample, the critical frequency being defined as the frequency at which the dispersion was 0.5. Based on this it seemed reasonable to assume that the dielectric dispersion curve might be related in some way to the molecular weight distribution curve in the case of a polydisperse sample. An attempt was thus made to obtain the dielectric dispersion curve in as much detail as possible, since this served the two-fold purpose of permitting conclusions to be drawn concerning the validity of the molecular weight relationship in dilute solutions and giving an indication of any influence of the molecular weight distribution on the dielectric dispersion. Apparatus was developed which gave a precision of 0.01 uuf in a total of about 250 uuf. This provided the means of measurement. Measurements were made on several fractions and three artificial blends. The results of measurements on fractions indicated that the form of the critical frequency-molecular weight relationship determined earlier (1) was correct. The dispersion curves obtained for the three blends of varying width indicated that the slope of the dispersion curve at the critical frequency decreased as the width of the blend was increased. This was the only indication of an influence of the width of the distribution on the dispersion curve. It was noted, however, that the slopes of the dispersion curves for the fractions were less than those for the narrowest blend in several cases. The most reasonable explanation of this observation seems to be that the fractions and blends have much wider distributions than was believed. It is also possible that chain flexibility might be a complicating factor. (1) Scherer, P. P., Levi, D. W., and Hawkins, M. C.: J. Polymer Sci., 24, 19 (1957) (2) Scherer, P. C., Hawkins, M.C., and Levi, D. W.: J. Polymer Sci., 31, 105 (1958) (3) Scherer, P. C., Hawkins, M. C., and Levi, D. W.: J. Polymer Sci., 37, 369 (1959) / Ph. D.

Additive Manufacturing (AM, also known as 3D printing) can produce novel three-dimensional structures using low-loss dielectric materials. This enables the construction of dielectrics with complex shapes that enable innovative microwave applications such as resonators, filters, and metamaterial lenses. This thesis addresses the production and characterization of cellular structures of various designed densities created with a low loss ceramic material, alumina (aluminum oxide), via vat photopolymerization. The permittivity of these printed structures is variable over roughly an octave, with a range of relative permittivites from 1.78 to 3.60, controlled via part geometry. Two additional materials, ferrite and nickel, have been explored for inclusion within these dielectric structures to enable the production of multi-material electromagnetic structures with conductive, magnetic, and dielectric elements. / Master of Science

A new model for the complex dielectric constant, ε, of aqueous ammonia (NH4OH) has been developed based on laboratory measurements in the frequency range between 2-8.5 GHz for ammonia concentrations of 0-8.5 %NH3/volume and temperatures between 277-297 K. The new model has been validated for temperatures up to 313 K, but may be consistently extrapolated up to 475 K and ammonia concentrations up to 20 %NH3/volume. The model fits 60.26 % of all laboratory measurements within 2σ uncertainty. The new model is identical to the Meissner and Wentz (2004) model of the complex dielectric constant of pure water, but it contains a correction for dissolved ammonia. A description of the experimental setups, uncertainties associated with the laboratory measurements, the model fitting process, the new model, and its application to approximating jovian cloud opacity for NASA's Juno mission to Jupiter are provided.

Since the development of the understanding that the electron distribution within a molecule is chiefly responsible for its properties and behavior, factors influencing this charge distribution have been of interest to scientists. The chemical reactivity of a molecule, the physical properties, and to a large extent, structure and geometry, are all functions of the electron distribution. This study examines the issue of electronic structure from two points of view, each of them focussing on a specific component within the molecules studied. In the present work, the effects of the highly polar carbonyl group on spectroscopic parameters and physical behavior are investigated. An additional area of study is the effect of fluorine substitution on the energy levels of some halogenated ethylenes. The specific parameters examined are the ionization potentials, the absorption frequencies, and the energies of a class of excited states known as molecular Rydberg states. It was during the study of these halogenated ethylenes that the observations leading to the carbonyl compound investigations were made, so that the two areas examined are connected both experimentally and chemically.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2006. / The open-ended coaxial probe is revisited as a broadband measurement system for general high frequency permittivity measurements. Three coaxial probes were developed that are suited for the measurement of both liquids and solids. The components of a permittivity measurement system were investigated and improvements were made to the coaxial probe where needed. This includes the development of a full wave code with great calculation time improvements without sacrificing accuracy. This code allows measurements to be performed in a high frequency laboratory and the permittivity extracted without any mentionable delay. A capacitance model that better describes the impedance of an open-ended coaxial line is also suggested that can be used for real-time permittivity extraction over a limited frequency range. Calibration formed a vital part of the project and great time was spent developing a TRL and a SOLT calibration set for the coaxial probe geometry. The combination of the TRL and SOLT standards also allows measurement of the residual errors after calibration and is used in an uncertainty analysis of the extracted permittivity. Well known materials such as PTFE, PVC, methanol and water were measured to test the probes. The measured dielectric constants are all within 3% of values quoted in literature. The loss term of the samples are also in good agreement with the expected values.

In recent years, both hafnia and zirconia have been looked at closely in the quest for a high permittivity gate dielectric to replace silicon dioxide in advanced metal oxide semiconductor field effect transistors (MOSFET). Hafnium dioxide or HfO2 is chosen for its high dielectric constant (five times that of SiO2) and compatibility with stringent requirements of the Si process. As deposited, thin hafnia films are typically amorphous but turn polycrystalline after a post-deposition anneal. To control the phase composition in hafnia films understanding of structural phase transitions is a first step. In this dissertation using first principles methods we consider three phase transitions of hafnia and zirconia: monoclinic to tetragonal, tetragonal to cubic and amorphous to crystalline. Because the high surface to volume ratio in hafnia films and powders plays an important role in phase transitions, we also study the surface properties of hafnia. We discuss the mechanisms of various phase transitions and theoretically estimate the transition temperatures. We find two types of amorphous hafnia and show that they have different structural and electronic properties. The small energy barrier between the amorphous and crystalline structures is found to cause the low crystallization temperature. Moreover, we calculate work functions and surface energies for hafnia surfaces and show the surface suppression of the phase transitions. / text

A irradiação de dielétricos sólidos com feixes eletrônicos tem-se revelado uma poderosa ferramenta na investigação das características de armazenamento e condução de carga elétrica nesses materiais. O presente trabalho faz urna revisão dos resultados que se tem obtido no campo, discute uma equação de transporte para carga em excesso em isolantes amorfos e aplica-la a particulares montagens experimentais. Encontra-se que os efeitos de carga espacial não são em geral importantes para pequenas penetrações do feixe na amostra, mas desempenham papel essencial no caso de elétrons quase-trespassantes. Em particular, obtém-se satisfatória concordância com os resultados experimentais de Spear (1955). / Irradiation of solid dielectric with electron beams has been used as a powerful tool in investigations of charge storage and transport in such materials. The present work reviews some of the results that have been obtained in this area and discusses the formulation of a transport equation for excess charge in irradiated insulators. This equation is subsequently applied to various experimental set-ups . It is found that space charge effects p1ay an essential role in the establishment of stationary currents in samples subject to quasi- -penetrating electron beams. Such effects may, however, be neglected for low electron ranges. Theoretical results are in good agreement with experimental findings by Spear (1955).